Protein tyrosine kinases (PTKs) play a critical role in the regulation of normal cell growth and differentiation; their overexpression may confer a growth advantage to breast cancer cells by increasing sensitivity to locally acting peptide growth factors, or by decreasing sensitivity to apoptotic signals. Elevated activity of membrane receptor PTKs occurs in a significant portion of breast tumors. Total soluble PTK activity in the cytosolic fractions of a majority of malignant human breast cancers is also higher than that from benign or normal breast tissue. Indeed, several novel non-receptor PTKs have recently been cloned from malignant human breast tissues and found to be highly active and/or overexpressed in a majority of breast cancers examined thus far. It is therefore important to define the role of these less well-characterized PTKs as novel components of known signaling pathways also overexpressed in a large proportion of human breast cancers. A novel nonreceptor PTK, termed breast tumor kinase (Brk) was cloned from a human metastatic breast tumor, and found to be overexpressed in human breast carcinomas and breast cancer cell lines, but not in normal adult breast tissue. Although clearly functionally distinct, Brk is closely related to c-Src, and contains one SH3-domain and one SH2-domain. In human breast cancer cells, Brk was activated in response to heregulin/c-erbB2 activation and Brk knock-down resulted in blockade of heregulin-induced ERK5 activation and cell proliferation. We hypothesize that Brk confers a growth and/or survival advantage to human breast cancer cells by acting as a signaling component downstream of erbB2/erbB3 receptor family members and upstream of ERK5, an emerging key regulator of cell growth, survival and transformation in mammary epithelial cells. Herein, we will 1) define the specificity and activity of a Brk-dependent signal transduction pathway to ERK5 activation in breast cancer cells 2) assay Brk-dependent alterations in cell biology and transformation in cells overexpressing Brk and during Brk gene-silencing, and 3) test the effects of Brk overexpression in the intact mammary gland in the context of heightened erbB2 signaling using transgenic mouse models. By identifying mechanisms underlying changes in growth factor receptor-mediated signaling events, we will increase the repertoire of regulatory proteins known to be involved in breast cancer cell growth and survival. Resistance to Herceptin is a frequent occurrence in patients with advanced breast cancer. Non- receptor PTK pathways acting downstream of erbB signaling may prove to be useful targets for chemotherapeutic intervention.